Analysis of rain-shadows in the Ethiopian Mountains using climatological model data

Orography is known to affect local meteorological conditions by inducing orographic rainfall and a rain shadow i.e. reduced rainfall on the mountain’s leeside with respect to the windward side. Therefore it has a strong effect on the local population and agriculture. Recent work highlights the ambiguities in the definition and difficulties in quantification of the rain shadow effect using observational data. A statistical approach is presented that allows its investigation based on climatological model data in geographically complex regions. This approach requires gridded rainfall and wind along with the model topography. The statistical aspects that contribute to the rainfall enhancement at the windward side are disentangled. These include, for windward and leeward events separately: frequency of occurrence, rainfall-event frequency, rainfall depth per event. By spatial aggregation the regional dependence of these statistics are calculated and visualized. The approach is used to characterize summer rain over the Ethiopian Highlands based a 21-year long simulation with the regional climate model ALARO-0 at 4 km resolution. There is an overall increased rainfall of 40% for windward events as compared to leeward events, but regionally this can exceed 150%. This increase can be attributed to the positive differences between windward and leeward events in their frequency of occurrence (on average 20%), and, in the rainfall per rainfall event (on average 16%). Mapped rain shadows correspond well to earlier qualitative observations and the small differences in probability of precipitation confirm that the mechanisms underlying the shadow effect are more complex than the textbook explanation.